280 The Phenomena of Morphogenesis 



leaves seems often to be due to nutritional disturbances. Bond (1945) 

 found that sepal phyllody in roses is an effect of hormones that control 

 the vigor of growth and the balance between reproductive and vegeta- 

 tive tendencies. This and other transformations of one sort of floral 

 organ into another may be compared to changes in sex expression that 

 have been found to occur after hormone treatment (Love and Love, 

 1946 ) or as the result of photoperiodic change. 



In other organoid galls, abnormality consists in the formation of struc- 

 tures in places where they do not ordinarily occur. Thus ovaries may 

 appear in normally staminate flowers, stamens within an ovary, or 

 ovules on its surface. In the well-known case of Lychnis vespertina at- 

 tacked by the smut fungus Ustilago antherorum, stamens are produced 

 in the female flower ( Strasburger, 1900). Flowers and cones may pro- 

 liferate into vegetative shoots after attack by mites or for other reasons. 

 Cecidomyia causes the formation of rootlets on the stalk nodes of Poa 

 (Beyerinck, 1885) and Eriophyces fraxini small, shortened shoots on 

 the leaves of Fraxinus. The attack by mites sometimes results in the 

 growing out (enation) of small leaves on large ones or the formation of 

 extra perianths. 



Familiar examples of abnormal development which may also be classed 

 as organoid galls are "witches'-brooms" (Fig. 11-4), dense clusters of 

 small, much-branched shoots, chiefly on woody plants and resulting from 

 excessive production of buds which grow immediately into shoots. At- 

 tacks by mites and various fungi, especially Exoascaceae and Uredineae, 

 are often the causal agents, though in many instances no parasites are 

 known to be involved. The physiological basis for the development of 

 such structures has been thought to be the accumulation of nutrients, 

 though doubtless there are other factors. 



A rather extensive literature has grown up about the character and 

 causation of witches'-brooms, for which the reader is referred to Solereder 

 ( 1905 ) and Liernur ( 1927 ) . Liernur cites 96 instances the causes of which 

 are known, occurring on 49 species of plants in 19 families, and 51 cases 

 of unknown origin. They differ in morphology, anatomy, and etiology but 

 resemble each other in their general character of copious, compact 

 branching. As compared with normal structures, the tissues of their leaves 

 and branches tend to be somewhat less highly differentiated, thus ap- 

 proaching cataplasmatic galls, though mechanical elements are often well 

 developed. 



Fasciations. A special type of abnormalities of a rather distinct kind 

 and which may be classed with organoid galls are fasciations. These are 

 cases where a normally cylindrical or radially symmetrical plant part be- 

 comes flattened and elliptical in cross section to form ribbon-like or some- 

 times ring-like structures. The origin of fasciation has attracted a good 



